Synthesis and Reactivity of Nickel and Palladium Fluoride Complexes with PCP Pincer Ligands. NMR-Based Assessment of Electron-Donating Properties of Fluoride and Other Monoanionic Ligands

Abstract

A series of complexes of the type [( iPr PCP)M-L] (M = Ni, Pd), containing the 2,6-bis(diisopropylphosphinomethyl)phenyl ( iPr PCP) pincer ligand and simple monoanionic ligands L (F, Cl, Br, H, Me, Ph, NO 3, and OTf), have been synthesized and characterized. The fluoride derivatives [( iPr PCP)M-F] were prepared from the halides [( iPr PCP)M-X] (M = Ni, X = Br; M = Pd, X = I) by exchange reactions with AgF or, alternatively, by protonolysis of the methyl complexes [( iPr PCP)M-Me] with Et 3 N·3HF (TREAT-HF). A survey of the 13 C NMR data for the new complexes and previously reported ones with L = OH, OMe, and NH 2 revealed significant trends that can be directly related to the electronic properties of the anionic ligands L coordinated to Ni(II) and Pd(II) centers. The chemical shift of the 13 C resonance of the metal-bound ipso carbon is very sensitive to the σ-donating capacity of L, which increases in the order L = OTf > NO 3 > F > OAc > OMe > Cl > OH ≈ Br > I > NH 2 > Ph > Me > H. In addition, the reactivity of [( iPr PCP)M-F] complexes was investigated. These readily undergo fluoride exchange with LiOMe, affording the corresponding methoxides [( iPr PCP)M-OMe] and LiF. They also react with n -C 12 H 13 I to cleanly yield [( iPr PCP)M-I] and n -C 12 H 13 F. Comparison of the rates of the latter reactions and analogous ones involving bromide or chloride complexes showed that the fluorides are significantly more nucleophilic, particularly the palladium derivative. This property was applied in a palladium-catalyzed reaction for the synthesis of alkyl fluorides using AgF as the fluoride source.